1. Field of the Invention
This invention relates generally to solder bonding, and more particularly to solder bonding for device packaging.
2. Description of the Related Art
Microelectromechanical systems (MEMS) are integrated micro devices or systems combining electrical and mechanical components. Some MEMS devices may be fabricated using standard integrated circuit batch processing techniques and have a variety of applications including sensing, controlling and actuating on a micro scale. MEMS devices may function individually or in arrays to generate effects on a macro scale.
Certain MEMS devices require a vacuum environment in order to obtain maximum performance. The vacuum package also provides protection in an optimal operating environment for the MEMS device. Examples of these MEMS devices are infrared MEMS such as bolometers and certain inertial MEMS such as gyros and accelerometers. Solder is often used as a sealing material for forming a vacuum package around a MEMS device. In a wafer bonding process, a lid wafer is aligned and mounted to a device wafer with an annular seal ring of solder to form an enclosed cell at each die location. This lid attachment process is completed in a vacuum environment, leaving each MEMS device in a vacuum cell. In this regard, wafer bonding for vacuum packaging of semiconductor devices requires soldering at low temperatures, normally 300 to 350° C. or lower. Past soldering methods include wetting and quenching of the solder, which can result in poor adhesion and high stress bonds.